Spatial characteristics and modulation of terahertz pulse from femtosecond laser air filament at high repetition rates

Terahertz source based on laser air plasma filamentation in air has a broad spectral bandwidth and strong electric field, no damage threshold limitations, which finds the applications in remote sensing, communications, biomedical and so on. The spatial properties of filament-based THz beams, particularly beam pointing stability, remain largely unexplored. The beam pointing of the THz generated from two-color laser filament in air was measured at the repetition rates up to kHz. Beam pointing jitter of the air filament-based THz increased by about four-fold when the laser repetition rate increased from 200 Hz to 1000 Hz. By applying an external DC electric field to the air filament, the beam pointing jitter was reduced by more than two-fold. Based on the fluid dynamics evolution induced by filamentation, the strong THz beam pointing jitter at the high repetition rates and its suppression by the external electric fields are both explained. The theoretical results indicate that the turbulence induced by high repetition rate gives rise to beam pointing jitter, and present a quantitative analysis of the correlation between the magnitude of turbulence and the intensity of jitter. This technique provides a way to generate a stable, high-repetition-rate THz source with broad bandwidth and strong field, which opens more opportunities for THz applications.